Recyclable removal of bisphenol A from aqueous solution by reduced graphene oxide–magnetic nanoparticles: Adsorption and desorption

Journal of Colloid and Interface Science 421 (2014) 85–92

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Recyclable removal of bisphenol A from aqueous solution by reduced graphene oxide–magnetic nanoparticles: Adsorption and desorption Yixuan Zhang a, Yuxiao Cheng b, Ningning Chen a, Yuyan Zhou b, Bingyu Li a, Wei Gu a, Xinhao Shi a, Yuezhong Xian a,⇑ a b

Department of Chemistry, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China Shanghai Entry-Exit Inspection and Quarantine Bureau, Shanghai 200135, China

a r t i c l e

i n f o

Article history: Received 7 November 2013 Accepted 16 January 2014 Available online 30 January 2014 Keywords: Reduced graphene oxide Magnetic nanoparticles Adsorption Desorption Bisphenol A

a b s t r a c t Reduced graphene oxide (rGO) nanosheets decorated with tunable magnetic nanoparticles (MNPs) were synthesized by a simple co-precipitation method and employed for recyclable removal of bisphenol A (BPA) from aqueous solution. The morphological characterization shows that Fe3O4 nanoparticles are uniformly deposited on rGO sheets. The magnetic characterization demonstrates that composites with various amounts of Fe3O4 nanoparticles are superparamagnetic. Due to the superparamagnetism, rGOMNPs were used as recyclable adsorbents for BPA removal in aqueous solution. The kinetics of the adsorption process and the adsorption isotherm were investigated. The results indicate that the adsorption process is fitted to Langmuir model and the composites with lower density of MNPs represent better adsorption ability. In addition, its kinetics follows pseudo-second-order rate equation. Moreover, the adsorbents could be recovered conveniently by magnetic separation and recyclable used because of the easy desorption of BPA. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Bisphenol A ((CH3)2C(C6H4OH)2, BPA) is widely used in the industrial production for polycarbonate polymers [1], epoxy resins [2] and other plastics. In recent years, BPA has received increasing concern because it is one kind of endocrine–disrupting chemicals (EDCs) which can mimic estrogen and lead to negative health effects on animals and human beings [3]. It has been reported that BPA can cause cancerous tumors, birth defects and other developmental disorders even at very low part-per-trillion doses [4]. The main sources of BPA in environmental water are expected to be the discharge of municipal effluent and industrial wastewater. In addition, BPA is non-biodegradable and highly resistant to chemical degradation [5]. In order to minimize harm for environment and human beings, cost-effective and high efficient technologies for rapid removal of BPA from wastewater are urgently required. Conventional technologies for BPA removal include adsorption [6], solvent extraction [7], membrane separation [8] and photodegradation [9]. Among them, the adsorption technique is considered as a more competitive method for BPA removal because of its simplicity, high efficiency and wide-ranging availability. Carbonaceous ⇑ Corresponding author. Fax: +86 21 54340046. E-mail address: [email protected] (Y. Xian). http://dx.doi.org/10.1016/j.jcis.2014.01.022 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

materials with high specific surface areas are well-known for their high adsorption capacity, and some of them, such as activated carbon [10], carbon nanotubes [11] have been reported as adsorbents for BPA removal. Graphene, a fascinating two-dimensional carbon material, has been reported with high conductivity, strong mechanical strength, and extremely high specific surface area [12]. Graphene based materials, including reduced graphene oxide (rGO) and functionalized graphene, have been applied in many fields with superior performances, such as catalysis [13], supercapacitors [14], photoelectric materials [15], and adsorbents [16]. Due to the large delocalized p-electron system, it has been identified that graphene can be used as a potent adsorbent for aromatic compounds [17–19] by p–p interactions. Recently, Xu and colleagues [20] reported the decontamination of BPA from aqueous solution using graphene as adsorbents. Both p–p interactions and hydrogen bonds might be responsible for the adsorption of BPA on graphene. However, the use of graphene on a large scale is limited by the difficulties of separation and regeneration. Graphene based adsorbents with high efficiency, low cost, convenient regeneration and easy separation are highly desired. Magnetic nanoparticles (MNPs) have been widely used as adsorbent with the advantage of high surface area and strong magnetic responsivity [21]. MNPs have been used as adsorbents for BPA removal. For example, molecularly imprinted polymer

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functionalized MNPs were reported to be used as absorbents for fast and selective removal BPA [22,23]. b-Cyclodextrin modified Fe3O4@SiO2 were synthesized via the combination of atom transfer radical polymerization and ring-opening reaction and the obtained nanocomposites revealed remarkably high adsorption capacity in the removal of BPA from aqueous solutions [24]. The separation and recycle of adsorbents can be easily manipulated by a simple magnetic process without the need of centrifugation or filtration. Therefore, the integration of MNPs with graphene could combine the high adsorption capacity of graphene and the separation convenience of magnetic materials. Meanwhile, the presence of graphene could effectively prevent the aggregation of MNPs and enable good dispersion of MNPs [25]. Fe3O4/SiO2 core/shell nanoparticles were attached to graphene oxide and used as adsorbents for methylene blue in aqueous solution [26]. Very recently, magnetic nanomaterial derived from graphene oxide and layered double hydroxide was successfully synthesized for efficient removal of methyl orange from aqueous solution [27]. Sinha et al. reported the synthesis of c-Fe2O3 nanoparticle modified graphene for the removal of EDCs from water. The c-Fe2O3 nanoparticles could partially inhibit graphene–graphene interaction and offer water dispersibility [28]. In our previous work, GO/Fe3O4 composites were synthesized by click reaction for recyclable removal of heavy metal ions from aqueous solution [29]. In addition, GO/Fe3O4 was modified on glassy carbon electrode for electrochemical measurement of BPA [30]. In the present study, rGO-MNPs with tunable loading of MNPs were synthesized for the removal of BPA from water samples. Firstly, rGO was synthesized by the reduction of GO with hydrazine hydrate. Then, rGO-MNPs were obtained by in situ chemical coprecipitation of Fe2+ and Fe3+ in alkaline solution in presence of rGO. The amount of MNPs on rGO was controlled by varying the weight ratio of rGO to ferric salt. Batch adsorption technique was used to study the adsorption and desorption of BPA on rGO-MNPs. The adsorption kinetics and thermodynamics were investigated, and the regeneration and reusability of the adsorbents were also evaluated. All results indicate that rGO-MNPs could be used as efficient, cost-effective and recyclable adsorbent for BPA removal. 2. Experimental 2.1. Materials Natural graphite (99.95% pure), ferric chloride hexahydrate (FeCl36H2O), ferrous chloride tetrahydrate (FeCl24H2O), sodium hydroxide, and hydrochloric acid were purchased from Sinopharm Chemical Reagent Company. BPA was purchased from TCI (Shanghai) and its stock solution (20 g L1) was prepared with ethanol. All the chemicals were used directly without further purification. All other chemicals and solvents were of analytical grade. Ultrapure water was used throughout the experiments. 2.2. Apparatus and measurements Surface morphological images of rGO and rGO-MNPs were recorded by a HITACHI S-4800 scanning electronic microscope (SEM) (Hitachi, Japan). Transmission electron microscopy (TEM) images were obtained at 200 kV by a JEM-2100F (JEOL, JAPAN) instrument. A drop of suspension of composites in ethanol was placed on a carbon-coated copper grid (Xinxin Bairui Corp, Beijing, China) and dried in air. X-ray powder diffraction (XRD) measurements were performed on a powder sample of GO, rGO and rGO-MNPs using a Rigaku D/Ultima IV X-ray diffractometer (Rigaku, JAPAN), which was operated at 35 kV and 40 mA at a scan rate of 0.4 deg s1 using Cu Ka radiation (k = 0.1542 nm). Raman spectro-

scopic measurements were performed using a confocal microprobe Raman system (LabRam II, Dilor, France). Thermogravimetric analysis (TGA) was carried out in air using a SDTA851 TGA analyzer (Mettler Toledo, Switzerland). The amount of iron oxide in the rGO-MNPs was measured from ambient temperature up to 800 °C at a heating rate of 10 °C min1. The vibrating-sample magnetization (VSM) curves of MNPs and rGO-MNPs were obtained by using a Quantum Design vibrating-sample magnetometer. Approximately 20 mg powder samples were put into a diamagnetic plastic straw and packed to a minimal volume for magnetic measurements. 2.3. Synthesis of rGO GO was synthesized according to the modified Hummer’s method [31]. Then, rGO was obtained by reducing GO with hydrazine hydrate. Briefly, GO (100 mg) was dispersed in 200 mL water and ultrasonicated for 3 h using an ultrasonic cleaner (180 W, 40 Hz), through which the bulk GO powders were transformed into GO sheets. The obtained brown dispersion was centrifuged at 3000 rpm for 30 min to remove any un-exfoliated GO. Then, 200 lL of hydrazine hydrate and 3 mL of ammonia solution were added to GO solution. After that, the mixture was stirred vigorously for a few minutes and refluxed at 80 °C for 12 h in an oil bath. The final products were centrifuged, washed, and finally dried under vacuum. 2.4. Synthesis of rGO-MNPs rGO-MNPs were synthesized by in situ chemical co-precipitation of Fe2+ and Fe3+ in alkaline solution in the presence of rGO. The molar ratio of Fe2+ and Fe3+ was 1:2. Firstly, 200 mg rGO, 172.2 mg FeCl24H2O and 280.95 mg FeCl3 were dissolved in 200 mL of deionized water. Then, 1.5 mL NaOH (10 mol L1) solution was added and the pH of the solution was adjusted over the range of 11–12. After that, the mixture was heated to 80 °C under continuous mechanical stirring for 1 h. The precipitate was separated by a permanent magnet and washed with double distilled water, then dried under vacuum at 60 °C. The obtained materials were referred as rGO-MNPs-1(w(rGO)/w(ferric salt) = 1:2). Through increasing iron salts in the co-precipitation process, two products referred as rGO-MNPs-2 (w(rGO)/w(ferric salt) = 1:3) and rGO-MNPs-3(w(rGO)/w(ferric salt) = 1:4) were obtained. 2.5. Adsorption The adsorption of BPA by rGO-MNPs was performed using batch equilibrium technique in aqueous solution. In general, 10 mg of rGO-MNPs was dispersed in 50 mL BPA solution with known initial concentration. The adsorption was carried out in a thermostatic bath with constant agitation (200 rpm) at different temperature ranging from 298 to 328 K. After equilibrium, the adsorbents were removed using a permanent magnet and the supernatant was collected. The concentration of BPA in the supernatant was quantified by measuring the absorbance of the solution using UV–vis spectrophotometer (UV-2550, SHIMADZU, Japan) at wavelengths of 276 nm. The adsorption was conducted with different pH from 3.0 to 10.0. The adsorption capacity (qe in mg g1) of BPA was calculated based on the difference in the BPA concentration in the aqueous solution before and after adsorption [32], according to the following equation:

qe ¼

ðC O  C e Þ  V m

ð1Þ

where Co and Ce are the initial and equilibrium concentration of BPA (mg L1), m is the weight of rGO-MNPs (g), and V is the volume of solution (L).

Y. Zhang et al. / Journal of Colloid and Interface Science 421 (2014) 85–92

Adsorption kinetics was investigated by placing 10 mg adsorbents in a series of flasks containing 20 mg L1 BPA solution (pH 6.0) at 298 K. At certain time intervals, the composites were removed from the solution through magnetic separation, and BPA in the solution was analyzed with UV–vis spectrophotometer. Adsorption isotherms of different adsorbents (rGO-MNPs-1, rGO-MNPs-2, rGO-MNPs-3) were obtained at 298 K with initial pH of 6.0. Each flask contained 10 mg adsorbents and the concentration of BPA varied from 10 to 180 mg L1. After oscillated for 6 h, the composites were removed with a magnet and the resultant solution was analyzed. 2.6. Desorption When the adsorption reached equilibrium, the adsorbents were collected from the suspension by magnetic separation. Then, the adsorbents were redispersed into 5 mL of desorption solvents, including methanol, ethanol, toluene and cyclohexane, respectively. After shaking for 30 min, the adsorbents were removed and the concentration of BPA in the supernatant was measured. Desorption efficiency of BPA was calculated as the ratio of the amount of the desorbed BPA to the amount of the BPA initially adsorbed. To test the reusability of the absorbent, the adsorption–desorption process was repeated for five times. 3. Results and discussion

nearly spherical and possess regular lattice fringes with d-spacing of 0.26 nm, which corresponds to the (3 1 1) planes of a spinelstructured iron oxide. The interlayer distance of stacked rGO is about 0.34 nm, corresponding to the spacing of the (0 0 2) planes of graphite [25]. The selected area electron diffraction (SAED) pattern of rGO-MNPs-1 (inset of Fig. 2) shows typical diffraction patterns of rGO and Fe3O4 nanoparticles. XRD measurements were carried out to investigate the phase structure of the obtained samples. The patterns of GO, rGO, and rGO-MNPs are shown in Fig. 3. The pattern of GO shows a peak at 2h = 10.4°, indicating the AB stacking is still observed in GO [33]. After reduced by hydrazine hydrate, a broad peak at 2h = 26.5° appears, owing to the (0 0 2) reflection of rGO [34]. The (0 0 2) reflection is very broad, indicating that free graphene nanosheets are very poorly ordered along the stacking direction [34]. As for different rGO-MNPs composites, the peaks at 2h of 30.3°, 35.7°, 43.5°, 53.5°, 57.5° and 62.9° can be assigned to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) of crystal planes of Fe3O4 (JCPDS No. 75-0033). In addition, a very weak and broad diffraction peak at 26.5°, which is corresponding to the (0 0 2) reflection of rGO, is also observed. It suggests that Fe3O4 nanoparticles deposited on the rGO surface could suppress the stacking of graphene layers. The combination of XRD data and TEM observation demonstrates the coexistence of MNPs and rGO in all composites [35]. The crystallite size of Fe3O4 nanoparticles on rGO sheets is calculated from the full-width at half maximum of the strongest reflection of the (3 1 1) using the Debye–Scherrer equation,

3.1. Characterization of rGO-MNPs

D¼ The morphology of the obtained samples was characterized by SEM and TEM (Fig. 1). In Fig. 1a–c, the crumpled silk wave-like rGO with MNPs are observed in all three samples, indicating the formation of composites of rGO-MNPs. As shown in Fig. 1a, MNPs with average diameters of ca. 10 nm are well dispersed on the planes of graphene nanosheets without agglomeration. Increasing the amount of ferric salts, the density of Fe3O4 nanoparticles on graphene also increases and leads to a slight aggregation. TEM images further confirm that MNPs are highly dispersed on the graphene (Fig. 1d–f). It should be noted that even after sonication for a long time for the preparation of TEM samples, the Fe3O4 nanoparticles still anchor on the graphene nanosheets, suggesting the strong interaction between Fe3O4 nanoparticles and rGO. The high resolution TEM (HRTEM) in Fig. 2 reveals that Fe3O4 nanoparticles are

87

Kk b cos h

ð2Þ

where D is the average particle size (nm) of the samples, k is the wavelength of Cu Ka (0.1541 nm), K is the constant, and h and b are the Bragg angle and full width at half maximum (radians), respectively. In this work, the average particle size for Fe3O4 nanoparticles is about 9.7 nm, which is well in agreement with the observation of SEM and TEM. The significant structural changes occur during the chemical reduction GO to rGO, which are reflected in their Raman spectra. The main features in the Raman spectra of carbons are the socalled D and G band. The D peak is related to the breathing modes of the six-atom rings and requires a defect for its activation, and the G peak corresponds to the first-order Raman-allowed E2 g phonon at the Brillouin zone center [36,37]. As shown in Fig. 4a, the

Fig. 1. SEM images of rGO-MNPs-1 (a), rGO-MNPs-2 (b) and rGO-MNPs-3 (c). TEM images of rGO-MNPs-1 (d), rGO-MNPs-2 (e) and rGO-MNPs-3 (f).

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Fig. 2. HRTEM image of rGO-MNPs-1 and the insets are the SAED patterns for MNPs.

Fig. 3. XRD patterns of GO (a), rGO (b), rGO-MNPs-1 (c), rGO-MNPs-2 (d) and rGOMNPs-3 (e).

Raman spectrum of GO shows two strong peaks at 1347 and 1593 cm1 that are corresponding to the defect induced D band and the first-order scattering of the E2g mode for G band, respectively [38,39]. The intensity ratio of the D over the G peak (ID/IG) is generally accepted to reflect the graphitization degree of carbonaceous materials and the defect density [40]. Compared with that of GO (0.83), the ID/IG value (1.10) of the rGO is increased due to the increase in the number of polyaromatic domains or highly defected carbon lattice after reduction [41]. It also suggests a decrease in the average size of the sp2 domains upon reduction of the GO [42]. In addition, the increment of ID/IG for rGO-MNPs-1(1.27), rGO-MNPs2(1.46) and rGO-MNPs-3(1.53) shows a substantial reduction in the content of the sp3-bonded carbon atoms and the oxidized molecular defects. Meanwhile, the D and G peaks of rGO-MNPs are blue-shifted to 1328 and 1583 cm1, indicating the reduction of GO and the interaction between rGO and MNPs [41]. Generally, the oxygen-containing groups distributed on the surface of rGO sheets could act as anchoring sites for iron ions through electrostatic attraction and then as a nucleation center for the

Fig. 4. Raman spectra of GO (a), rGO (b), rGO-MNPs-1 (c), rGO-MNPs-2 (d) and rGOMNPs-3 (e).

Fig. 5. TGA curves of rGO (a), rGO-MNPs-1 (b), rGO-MNPs-2 (c) and rGO-MNPs-3 (d).

Y. Zhang et al. / Journal of Colloid and Interface Science 421 (2014) 85–92

nanoparticles growth. It enables the uniform deposition of Fe3O4 nanoparticles on rGO. TGA curves for rGO and rGO-MNPs were measured from 25 to 800 °C in air atmosphere with a heating rate of 10 °C/min (Fig. 5). Based on the weight loss upon graphene combustion and the assumption of Fe3O4 fully oxidized to Fe2O3 at around 400 °C in static air [43], the content of Fe3O4 in hybrid was calculated to be 61.7 wt%, 67.5 wt% and 72.7 wt% for rGO-MNPs-1, rGO-MNPs-2, rGO-MNPs-3, respectively. It should be noted that the decomposed temperature of the graphene in the rGO-MNPs decreased with the increase in Fe3O4, which might be caused by the interaction between graphene skeleton and Fe3O4 nanoparticles [44]. The magnetization curves of Fe3O4 nanoparticles and rGO-MNPs were measured at room temperature (Fig. 6). S-shaped hysteresis-loops without remanence are observed for all samples, indicating the superparamagnetic behaviors of rGO-MNPs [45]. The saturation magnetization values of Fe3O4 nanoparticles and rGO-MNPs-3, rGO-MNPs-2, rGO-MNPs-1 are 71.88, 51.25, 40.63 and 36.25 emu g1, respectively. For rGO-MNPs-1, the reduction in the saturation magnetization might be due to the low amount of MNPs loading on rGO, however, this magnetization is strong enough for magnetic separation (as shown in inset of Fig. 6). 3.2. Adsorption study The adsorption isotherm models are usually used to investigate the interaction between the absorbent and the adsorbate when the adsorption process reaches equilibrium. The equilibrium isotherms for the adsorption of BPA by various rGO-MNPs at 298 K are shown in Fig. 7. With the increasing initial concentration of BPA, the equilibrium adsorption capacity for BPA increases gradually until the equilibrium condition is reached. Langmuir and Freundlich isotherm models are employed to investigate the adsorption process. The Langmuir isotherm model assumes monolayer adsorption on a surface with a finite number of identical sites, that all sites are energetically equivalent and that there is no interaction between adsorbed molecules [46]. The Langmuir equation is expressed as Eq. (3),

Ce Ce 1 ¼ þ qe qm qm K L

ð3Þ

where Ce represents the equilibrium concentration of BPA in solution (mg L1), qe is the adsorption capacity at equilibrium state (mg g1), qm is the theoretical maximum adsorption capacity

Fig. 6. Magnetic hysteresis loops of Fe3O4 nanoparticles (a), rGO-MNPs-3 (b), rGO-MNPs-2 (c) and rGO-MNPs-1 (d). The inset photograph shows the magnetic responsive performance of rGO-MNPs-1 after adsorption of BPA in aqueous solution.

89

Fig. 7. Adsorption isotherms of BPA on various rGO-MNPs at 298 K and pH 6.0. The solid lines are the Langmuir model simulation, and the dotted lines are the Freundlich model simulation.

(mg g1), and KL is the Langmuir isotherm constant which is related to the affinity of binding sites (L mg1). High regression correlation coefficients (>0.97) for all rGO-MNPs (as shown in Table 1) demonstrate that the Langmuir model is applicable. Freundlich equilibrium isotherm is an empirical equation (shown as Eq. (4)) which is used for the description of multilayer adsorption with interaction between adsorbed molecules.

lnqe ¼

1 lnC e þ lnK F n

ð4Þ

where KF and 1/n are the Freundlich constants related to the adsorption capacity and intensity, respectively. The Freundlich constant (1/n) reflects the adsorption intensity of adsorbent and the magnitudes of 1/n lying between 0 and 1 suggest the adsorption is favorable [47]. The value of n is 5.04, 3.22, 2.88 for rGO-MNPs1, rGO-MNPs-2 and rGO-MNPs-3, respectively. It is generally stated that values of n in the range 2–10 represent good, 1–2 moderately difficult, and less than 1 poor adsorption, respectively [26]. The experimental data imply that the adsorption of BPA on all rGO-MNPs is favorable. According to the data of correlation coefficient (R2), the adsorption behavior fitted by Langmuir model is better than that of Freundlich model. In other words, the adsorption of BPA by rGO-MNPs is a monolayer adsorption manner. With regard to the saturated adsorption capacities, the rGO-MNPs-1 exhibits more effective adsorption, which might ascribe to the larger surface areas because of lower loading of MNPs. Therefore, rGO-MNPs-1 is selected as the adsorbent for BPA for further studies. The pH value of sample solution is an important factor in the adsorption process. The effect of initial solution pH on BPA adsorption was investigated over the pH range from 3 to 10.0 with initial BPA concentration of 20 mg L1. There are two factors that might influence the adsorption of BPA. One is p–p interaction between the benzene-ring of BPA and the skeleton of rGO, because BPA contained p electrons, which could interact with the p electrons of benzene rings of rGO. The other is hydrogen-bond interaction between the hydroxyl groups of adsorbent and BPA [20]. As shown in Fig. 8, the adsorption of BPA on rGO-MNPs-1 exhibits little pH dependence over the range of 3.0–6.0. It indicates p–p binding might be the main driving force for adsorption. Based on the molecular dynamics simulations, in the lowest energy configuration, the two phenol rings in the BPA are equally oriented at an angle to the surface of the graphene to maximum p–p interactions [48]. While the pH value exceeds 8.0, the deprotonation of BPA (pKa = 9.59) is realized [49]. Therefore, the electrostatic repulsion between BPA and rGO-MNPs results in the decrease in adsorption

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Table 1 Adsorption isotherm parameters for BPA on various rGO-MNPs at 298 K and pH 6.0. Adsorbent

rGO-MNPs-1 rGO-MNPs-2 rGO-MNPs-3

qe,exp (mg g1)

123.2 115.9 98.38

Langmuir isotherm

Freundlich isotherm 2

KL (L/mg)

qm (mg/g)

R

0.1547 0.07646 0.05878

125.0 123.5 106.4

0.9959 0.9948 0.9934

KF

n

R2

47.28 26.54 18.52

5.035 3.223 2.879

0.8821 0.9640 0.9334

rate constant of pseudo-first-order equation. Another kinetic model is pseudo-second-order model, which is expressed by Eq. (6),

t 1 t ¼ þ qt k2 q2e qe

Fig. 8. Effect of initial pH on BPA adsorption on rGO-MNPs-1.

capacity. In order to obtain high adsorption capacity, the sample solution was adjusted to pH 6.0.

ð6Þ

where k2 is the rate constant of pseudo-second-order equation. The rate constant and the correlation coefficient of each model are listed in Table 2. To evaluate the suitability of different models, the correlation coefficient R2 is introduced. Based on the comparison of the two models, the pseudo-second-order kinetic model fits the experimental data more accurately. In addition, the qe value obtained from pseudo-second-order model is more close to the experimental data. All the results confirm that the adsorption phenomena follow the second-order kinetics, suggesting the sorption is dependent on the amount of the solute adsorbed on the surface of adsorbent and the amount adsorbed at equilibrium [50,51]. The higher adsorption rate constant k2 (0.095 g mg1 h1) of rGO-MNPs-1 from the pseudo-second-order model than that of the pure graphene (0.033 g mg1 h1) [20] demonstrates faster removal rate of the rGO-MNPs-1. 3.4. Adsorption thermodynamics

3.3. Adsorption kinetics analysis The adsorption behavior of the rGO-MNPs-1 toward BPA at different time intervals is shown in Fig. 9. The results indicate that a fast adsorption process occurs during the first 2 h and reaches equilibrium within 4 h. The kinetic analysis was performed to investigate the mechanism of adsorption. The pseudo-first-order kinetic model is expressed by Eq. (5),

lnðqe  qt Þ ¼ ln qe  k1 t

ð5Þ

where qe and qt are the amount absorbed on per unit mass of adsorbent at equilibrium and at time t (h), respectively. k1 is the

It is well established that temperature is an important factor influencing the adsorption process. The adsorption of BPA on rGO-MNPs-1 was investigated at temperature of 298, 313 and 328 K. As shown in Fig. 10, the adsorption capacity of rGO-MNP1 decreases with the increasing temperature, indicating that a lower temperature is suitable for the adsorption of BPA. This phenomenon indicates that BPA sorption on rGO-MNPs-1 is an exothermic process. It is in agreement with the reports on sorption tetrabromobisphenol A from solution by GO [51] and carbon nanotubes [52]. The thermodynamic parameters related to the adsorption process can be derived from the van’t Hoff equation, 



lnK ¼ 

Fig. 9. Time-dependent adsorption of BPA by rGO-MNPs-1. The solid line is the fitting plot using pseudo-second-order model, and the dashed line is the fitting plot using pseudo-first-order model. The initial concentration of BPA was fixed at 20 mg L1.

DH DS þ RT R



ð7Þ

where R is the universal gas constant (8.314 J mol1 K1), T is the solution temperature in Kelvin, and K° is the thermodynamic equilibrium constant. A plot of lnK° against 1/T yields a straight line, and the values of DH° and DS° can be obtained from the slope and intercept. The thermodynamic parameters at different temperature are summarized in Table 3. The negative value of DH° also indicates the exothermic nature of adsorption, which is identified by the decrease in BPA adsorption capacity at high temperature. The negative value of DS° implies a decrease in randomness by the adsorbed species and indicates the stability of adsorption process with no structural change at solid–liquid interface [53]. Gibb’s free energy change, DG°, is the fundamental criterion of spontaneity reactions occur spontaneously at a given temperature if DG° is a negative value. The change in free energy was calculated from Eq. (8), 

DG ¼ RTlnK



ð8Þ

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Table 2 Adsorption rate constants for two kinetic models at 298 K and pH 6.0. Initial concentration Co (mg/L)

Pseudo-first-order model k1 (h1)

qe(cal) (mg/g)

R

k2 (h1)

qe(cal) (mg/g)

R2

20

0.695

18.475

0.728

0.095

78.74

0.9988

2

Pseudo-second-order model

Fig. 11b. Recyclable adsorption of BPA by GO-MNPs-1.

exothermic nature of the reaction exceeded the effect of increasing the diffusion rate as the temperature increased [55]. Generally, the values of the DG° are in the range of 0 to 20 kJ mol1 and 80 to 400 kJ mol1 for physical and chemical adsorption, respectively [52]. In this study, the values of DG° are in the range of 5.9 to 13.0, indicating the sorption process is mainly physical in nature. Fig. 10. Equilibrium isotherms for BPA on rGO-MNPs-1 at different temperature. The experimental data are fitted by Langmuir model (solid lines). Table 3 Thermodynamic parameters for the adsorption of BPA by rGO-MNPs at pH 6.0. Temperature (K)

lnK°

DG° (kJ mol1)

DH° (kJ mol1)

DS° (J mol1 K1)

298.15 313.15 328.15

5.264 3.668 2.154

13.05 9.55 5.877

84.2 84.2 84.2

238.52 238.52 238.52

3.5. Desorption and regeneration Considering cost-effective application of rGO-MNPs in wastewater treatment, the possibility of regeneration and reusability was further investigated. Desorption of BPA in different organic solvents was studied. The adsorbents were separated through magnetic field, and 5 mL organic solvent was used to regenerate the adsorbents. As shown in Fig. 11a, the desorption efficiency of methanol is about 98%. Thus methanol was selected for the regeneration of the rGO-MNPs. The regenerated adsorbents were reused for recyclable BPA adsorption and the data are shown in Fig. 11b. The results show that rGO-MNPs can be recycled at least five times without the expense of adsorption capacities. After five cycles, the adsorption capacity is around 95%. 4. Conclusion

Fig. 11a. The effect of eluting solvent on the desorption efficiency. (Adsorbent: 10 mg rGO-MNPs-1, initial BPA concentration: 20 mg L1, volume of solvent: 5 mL, pH: 6.0, temperature: 298 K).

The negative values of DG° confirm the thermodynamic feasibility of the sorption process and the spontaneous nature of adsorption with a high affinity of BPA by adsorbent [54]. Moreover, the increase in the value of DG° with an increase in temperature indicates that the adsorption process of BPA on rGO-MNP-1 becomes more favorable at lower temperatures. It also suggests the

In summary, the superparamagnetic rGO with tunable loading of MNPs was successfully synthesized by in situ and facile co-precipitation approach. Owing to the high specific areas of rGO and the magnetic properties of MNPs, the composites show excellent performance for BPA adsorption. The strong p–p interaction is the main drive force for the adsorption. The loading amount of MNPs has a great influence on adsorption performance. Kinetics studies reveal that the adsorption process is well-fitted by pseudo-second-order model. The spontaneous and exothermal nature of the adsorption process indicates the adsorption is more favorable at ambient temperature. In addition, the rGO-MNPs can be easily manipulated by an external magnetic field and exhibits excellent reproducibility and reusability. These results suggest that the composites have promising application in the removal of BPA from wastewater due to the efficient and fast adsorption as well as the easy magnetic separation and regeneration. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21175046), General Administration of Quality Supervision, Inspection and Quarantine of China (Nos. 2011IK041,

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2013IK017 and 2012IK048) and Open Foundation of Shanghai Key Laboratory of Green Chemistry and Chemical Process. References [1] C.F. Abrams, K. Kremer, Macromolecules 36 (2003) 260. [2] S. Ahmada, A.P. Guptab, E. Sharmina, M. Alama, S.K. Pandey, Prog. Org. Coat. 54 (2005) 248. [3] W.V. Welshons, K.A. Thayer, B.M. Judy, J.A. Taylor, E.M. Curran, F.S. vom Saal, Environ. Health Perspect. 111 (2003) 994. [4] L.N. Vandenberg, R. Hauser, M. Marcus, N. Olea, W.V. Welshons, Reprod. Toxicol. 24 (2007) 139. [5] W.L. Guo, W. Hu, J.M. Pan, H.C. Zhou, W. Guan, X. Wang, J.D. Dai, L.C. Xu, Chem. Eng. J. 171 (2011) 603. [6] B. Pan, D.H. Lin, H. Mashayekhi, B.S. Xing, Environ. Sci. Technol. 42 (2008) 5480. [7] B. Shao, H. Han, D.M. Li, Y.L. Ma, X.M. Tu, Y.I. Wu, Food Chem. 105 (2007) 12361. [8] J.H. Chen, X. Huang, D.J. Lee, Process Biochem. 43 (2008) 451. [9] V.M. Mboula, V. Héquet, Y. Andrès, L.M. Pastrana-Martínez, J.M. DoñaRodríguez, A.M.T. Silva, P. Falaras, Water Res. 47 (2013) 3997. [10] G.F. Liu, J. Ma, X.C. Li, Q.D. Qin, J. Hazard. Mater. 164 (2009) 1275. [11] J. Qu, Q. Cong, C.Q. Luo, X. Yuan, RSC Adv. 3 (2013) 961. [12] A.K. Geim, K.S. Novoselov, Nature Mater. 6 (2007) 183. [13] C.C.H.H. Bai, C. Li, G.Q. Shi, Chem. Commun. 47 (2011) 4962. [14] Y. Wang, Z.Q. Shi, Y. Huang, Y.F. Ma, C.Y. Wang, M.M. Chen, Y.S. Chen, J. Phys. Chem. C 113 (2009) 13103. [15] Z.Y. Liu, D.W. He, Y.S. Wang, H.P. Wu, J.G. Wang, Sol. Energy Mater. Sol. Cells 94 (2010) 1196. [16] S. Pavagadhi, A.L.L. Tang, M. Sathishkumar, K.P. Loh, R. Balasubramanian, Water Res. 47 (2013) 4621. [17] C. Wang, C. Feng, Y.J. Gao, X.X. Ma, Q.H. Wu, Z. Wang, Chem. Eng. J. 173 (2011) 92. [18] O.G. Apul, Q.L. Wang, Y. Zhou, T. Karanfil, Water Res. 47 (2013) 1648. [19] F. Zhang, B. Zheng, J.L. Zhang, X.L. Huang, H. Liu, S.W. Guo, J.Y. Zhang, J. Phys. Chem. C 114 (2010) 8469. [20] J. Xu, L. Wang, Y.F. Zhu, Langmuir 28 (2012) 8418. [21] J.H. Gao, H.W. Gu, B. Xu, Acc. Chem. Res. 42 (2009) 1097. [22] N. Griffete, H. Li, A. Lamouri, C. Redeuilh, K. Chen, C.Z. Dong, S. Nowak, S. Ammar, C. Mangeney, J. Mater. Chem. 22 (2012) 1807. [23] Y. Li, X. Li, J. Chu, C.K. Dong, J.Y. Qi, Y.X. Yuan, Environ. Pollut. 158 (2010) 2317. [24] Y. Kang, L.L. Zhou, X. Li, J.Y. Yuan, J. Mater. Chem. 21 (2011) 3704. [25] A. Prakash, S. Chandra, D. Bahadur, Carbon 50 (2012) 4209. [26] Y.J. Yao, S.D. Miao, S.M. Yu, L.P. Ma, H.Q. Sun, S.B. Wang, J. Colloid, Interface Sci. 379 (2012) 20.

[27] Z. Yang, S.S. Ji, W. Gao, C. Zhang, L.L. Ren, W.W. Tjiu, Z. Zhang, J.S. Pan, T.X. Liu, J. Colloid Interface. Sci. 408 (2013) 25. [28] A. Sinha, N.R. Jana, Chem. Asian J. 8 (2013) 786. [29] W.J. Zhang, X.H. Shi, Y.X. Zhang, W. Gu, B.Y. Li, Y.Z. Xian, J. Mater. Chem. A 1 (2013) 1745. [30] Y.X. Zhang, Y.X. Cheng, Y.Y. Zhou, B.Y. Li, W. Gu, X.H. Shi, Y.Z. Xian, Talanta 107 (2013) 211. [31] W.S.J. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [32] H.L. Guo, X.F. Wang, Q.Y. Qian, F.B. Wang, X.H. Xia, ACS Nano 3 (2009) 2653. [33] Z.M. Zhu, F.Q. Sun, L.T. Yang, K.Y. Gu, W.S. Li, Chem. Eng. J. 223 (2013) 395. [34] A.V. Murugan, T. Muraliganth, A. Manthiram, Chem. Mater. 21 (2009) 5004. [35] Y. Chen, B.H. Song, X.S. Tang, L. Lu, J.M. Xue, J. Mater. Chem. 22 (2012) 17656. [36] A.C. Ferrari, Solid State Commun. 143 (2007) 47. [37] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [38] Y. Li, J. Chu, J.Y. Qi, X. Li, Appl. Surf. Sci. 257 (2011) 6059. [39] Y. Liu, P.Y. Wu, ACS Appl. Mater. Interfaces 5 (2013) 3362. [40] G.Q. Xie, P.X. Xi, H.Y. Liu, F.J. Chen, L. Huang, Y.J. Shi, F.P. Hou, Z.Z. Zeng, C.W. Shao, J. Wang, J. Mater. Chem. 22 (2012) 1033. [41] N. Wu, X.L. She, D.J. Yang, X.F. Wu, F.B. Su, Y.F. Chen, J. Mater. Chem. 22 (2012) 17254. [42] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y.Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558. [43] C.D. Wang, Q.M. Zhang, Q.H. Wu, T.W. Ng, T.L. Wong, J.G. Ren, Z.C. Shi, C.S. Lee, S.T. Lee, W.J. Zhang, RSC Adv. 2 (2012) 10680. [44] D. Zhou, T.L. Zhang, B.H. Han, Micropor. Mesopor. Mater. 165 (2013) 234. [45] B.J. Li, H.Q. Cao, J. Shao, M.Z. Qu, J.H. Warner, J. Mater. Chem. 21 (2011) 5069. [46] X.G. Luo, L.N. Zhang, J. Hazard. Mater. 171 (2009) 340. [47] F. Yu, J. Ma, Y.Q. Wu, Front. Environ. Sci. Eng. 6 (2012) 320. [48] L.K. Boateng, J.Y. Heo, J.R.V. Flora, Y.G. Park, Y. Yoon, Sep. Sci. Technol. 116 (2013) 471. [49] M. Wu, B. Pan, D. Zhang, D. Xiao, H. Li, C. Wang, P. Ning, Chemosphere 90 (2013) 782. [50] Y. Zhang, J.Y. Hu, G.Z. Li, C. Christel, A. Pierre, Environ. Sci. 31 (2010) 1513. [51] Y.H. Zhang, Y.L. Tang, S.Y. Li, S.L. Yu, Chem. Eng. J. 222 (2013) 94. [52] I.I. Fasfous, E.S. Radwan, J.N. Dawoud, Appl. Surf. Sci. 256 (2010) 7246. [53] A.Z.M. Badruddoza, A.S.H. Tay, P.Y. Tan, K. Hidajat, M.S. Uddin, J. Hazard. Mater. 185 (2011) 1177. [54] M. Irama, C. Guo, Y.P. Guan, A. Ishfaq, H.Z. Liu, J. Hazard. Mater. 181 (2010) 1039. [55] Y.J. Feng, Z.H. Zhang, P. Gao, H. Su, Y.L. Yu, N.Q. Ren, J. Hazard. Mater. 175 (2010) 970.